The use of navigation receivers becomes increasingly pervasive in everyday life. It is all the more common that cars' on-board electronics, smart phones, tablets include navigation receivers, and that applications running thereon capture as input information on position and trajectory of the user of the terminal.
Navigation receivers rely on L-Band RF (radio frequency) signals transmitted by Medium Earth Orbiting satellites, which are generally included in constellations comprising tens of them to cover most of the surface of the earth, such as the GPS™ (US), Galileo™ (Europe), Glonass™ (Russia) and Beidou™ (China). These constellations are designated under the generic acronym of GNSS (Global Navigation Satellite System).
GNSS carrier signals are modulated by a pseudo-random code and a navigation message which allow calculation of a pseudo-range between the receiver and a definite satellite. With a minimum of four pseudo-ranges, it is possible to calculate Position, Velocity and Time (PVT) of the receiver. In receivers of the type which are used by consumers, the position information is the one which is directly used to compute the navigation solution.
PVT measurements are affected by a number of errors, some of which are intrinsic to the principle of measurement used (i.e, due to the deviation in trajectory of the RF signals through the atmosphere—ionosphere and troposphere—due to variations in the orbits of the satellites), intrinsic to the receiver and satellites imperfections (clock biases for instance), or intrinsic to some configurations of the satellites in view at a moment in time (i.e. elevation of the satellites over the horizon; low dispersion of visible satellites—high Dilution Of Precision or DOP). A number of corrections can be used to mitigate these errors, either with the use of specific processing techniques which are only available to certain types of receivers (i.e. bi-frequency receivers can mitigate ionospheric errors up to a gain of precision from a few tens of meters to a few meters or better, depending on the quality of the correctionso. This is known as Precise Point Positioning or PPP. Differential GPS (DGPS) or Real Time Kinematics (RTK) provide a precision of a few tens of centimetres thanks to an integration of outside information (relative positioning vis-à-vis a number of fixed reference stations with known positions).
It is more difficult to mitigate in a consistent and efficient manner some errors which depend on the position of the receiver, notably when this position is environed by a number of objects which reflect the navigation RF signals and/or mask a number of the satellites which should be in line of sight (LOS) at a moment in time. In such conditions, the precision of the calculation of the PVT may be quite poor, all other causes of errors being equal, both at the time of acquiring a GNSS signal and at the time of tracking said signal.
In urban canyons (i.e. streets in between skyscrapers), multipath will not only increase the error in the determination of the pseudo-range of a satellite (User Equivalent Range Error or UERE), but also the (Geometric) Dilution Of Precision, (GDOP or DOP), because the field of view of the antenna will be narrower thus limiting the increase in precision due to the use of additional satellites.
The degradation in UERE is due to the signal impairments of the definite satellite which is acquired or tracked by a tracking loop. Tracking of a satellite relies on a maximization of a correlation function between the acquired code signal and a number of local replicas generated by the receiver of the code signals which are specific to each satellite. The correlation functions will be corrupted by multipath and the satellite may be not correctly acquired or may be lost. Even if signal tracking is still achievable, signal impairments will affect the correlation function's shape, thus degrading the pseudo-range estimation, and the UERE.
Most of the mitigation techniques therefore rely on specific processing applied at the correlation stage. For a general overview of these prior art techniques which can be applied, see for instance Bhuiyan et alii, Advanced Multipath Mitigation Techniques for Satellite-Based Positioning Applications, International Journal of Navigation and Observation, Volume 2010, Article ID 412393. Among these techniques are: using narrow correlators (i.e. correlators spaced by much less than a chip—or code length); using numerous correlators per tracking channel instead of a standard number of three and performing a double delta calculation; Multipath Estimation Delay Lock Loop, which uses the output of the correlation functions to estimate a full navigation signal of a definite satellite comprising LOS and non LOS signals.
All these solutions are complex and costly to implement either at the hardware level or at the software level. In any case, they cannot be currently implemented in a standard consumer receiver of the type used in a smart phone or a car navigation system.
Other attempts at mitigating multipath have been tried, for instance solutions of the type described by Neuenschwander, “Satellite Tracking from Camera: The Inverse GPS Problem”, Swiss Federal Institute of Technology, Zurich, 2009. In approaches of this kind, images of the environment of the receiver are acquired; the images are converted into Earth Centered Earth Fixed coordinates using the output of an Inertial Navigation System (INS) which is fixed in relation to the GNSS receiver, and the satellites in LOS are deducted from a comparison between the output of the image processing and the satellites ephemerides which give positions of the satellites of a constellation as a function of time at a position.
Solutions of this kind are quite complex to implement because they require an INS platform and external data and necessitate significant processing power.
It is therefore important to find a solution to mitigate the effects of multipath which could be implemented in a standard receiver available to the general public.
The present invention discloses a solution to overcome the previously cited drawbacks.